1. Field of the Invention
This invention relates to a method and apparatus for selectively activating a secondary power source, such as a backup battery, for an electronic circuit. In particular, methods and apparatus for activating an on-board backup battery to retain the contents of electronic memory are disclosed wherein the battery remains disconnected during manufacture and shipment, and is automatically connected when the corresponding circuit board is placed in service.
2. Description of the Prior ARt
Many common forms of electronic memory must be periodically refreshed or otherwise require a source of electrical energy to maintain the memory contents during quiescent periods. To prevent loss of memory contents during periods when the primary power supply to the electronics fails, it is common to provide a separate power source for the memory retention function. In general, the power required to retain the contents of electronic memory is less than the power required to otherwise operate or update the memory. Thus a substantially smaller, lower capacity power source is generally sufficient to retain the memory contents, and is typically provided by relatively small disposable or rechargeable batteries. For convenience, these are typically incorporated onto to the circuit board or into the electronics module including the memory device itself.
Although the backup power requirement for memory devices is typically small, it is possible for the backup battery to be depleted by long-term use. This is a particular concern where the battery is permanently connected and the circuit board is not immediately installed in a host system or attached to a primary power supply. A significant portion of the total energy available from the battery may be wasted by unnecessarily powering the memory retention function between the time of manufacture and first use, or for extended periods in general. While the battery may not be fully depleted during such periods, less capacity will remain for beneficial use after the electronics have been installed in a host system or otherwise put into use. For example, a lithium battery may be able to continuously power the memory retention function of memory registers of a typical microprocessor for two years or more. However, if the circuit is not installed for use for one year, approximately one half of the usable battery power may be depleted before the circuit is put to use. Ultimate exhaustion of the backup battery may therefore occur sooner than if a "fresh" battery were in place when the electronics were installed.
Because failure of the backup power source may result in loss of important programming and data information, it is highly desirable to minimize the chance for backup battery failure or exhaustion, and to lengthen the shelf an service life of the battery.
Various techniques have been used to provide some level of safeguard against premature failure. For example, it is common to couple a backup power source to a memory device only during periods of failure of a primary power source. Various isolation devices (such as diode coupling) may be employed for this purpose, and modern memory and microprocessor devices typically provide for isolation of the backup power source while the primary power source is active. Although effective in reducing current drain on the backup battery and thus extending its life, these techniques are not effective to guard against premature depletion when the associated circuit is disconnected from a source of primary power for extended times, such as prior to installation of a circuit board into a host system.
To overcome this problem, removable batteries may be employed so that a fresh backup battery is installed only when the associated circuit board or module is installed in a host system or otherwise configured for operation. However, this technique has the disadvantage of requiring the installer to maintain an inventory of suitable batteries, and requires an additional step during installation. If the installer does not have an appropriate battery available, or otherwise fails to install the battery or to install it correctly, the backup function will not be available and data loss may therefore occur.
Switches or jumpers on the board may similarly be used to selectively connect the backup battery only when desired. However, there remains the danger that the switch or jumper is not properly set when the electronics are installed (resulting in potential loss of data), or that they are erroneously set to connect the battery during quiescent shelf storage of the electronics (depleting the battery prematurely).
Finally, numerous techniques have been developed for providing a rechargeable backup power source. In this manner, the battery is recharged while the primary source is connected, and therefore may retain a higher percentage of its available capacity for a longer period of time. However, rechargeable batteries typically have a lower power density than comparably sized disposable batteries (such as lithium batteries), and require additional circuitry to provide the recharging function. Further, the battery must be initially charged before installation if the backup is to be effective upon initial activation of the associated circuitry, requiring yet another processing step. Finally, even a fully charged battery my be depleted or fail prematurely if the associated circuits are not connected to a source of primary power for an extended period of time, such as after manufacture and before first use.